Internal polling is the most accurate because the campaigns only use sampling they KNOW to be true. To do otherwise would simply be lying to themselves and then using the flawed results to plan stratagy.
Even liberals aren’t that stupid
Never overestimate the intelligence of the loony left. I've never been disappointed with that rule of thumb.
The real lesson, however, is that the public polls are worthless.
What you said would be true, if everyone were rational and honest.
However leftists tend to be neither rational nor honest.
Additionally, it's human nature for most people to tell the boss what they think he wants to hear.
Pundits on FoxNews today say that both Obama and Romney camps claim their internal polls show their guy is winning. I think it is likely one side is being either irrational or dishonest. (And I think I know which side that probably is.)